Insect pathogenic nematodes of the families Steinernematidae and Heterorhabditidae are known to be symbiotically associated with bacteria of the genera Xenorhabdus and Photorhabdus respectively. It has been observed that these bacteria have the ability to kill a wide range of different insects without the aid of their nematode partners. The present inventors have isolated polynucleotide molecules encoding a new class of protein insecticidal toxins from Xenorhabdus nematophilus strain A24 and Photorhabdus luminescens strain V16/1.